It is known to use the compounds from the group of hexacyanoferrates to remove different isotopes of heavy metal ions of the elements cesium, strontium, thallium, cadmium and lead from aqueous systems. The hexacyanoferrates used are advantageously immobilized in a supporting material so as to prevent them passing into the aqueous or moist gaseous phase surrounding the material. Known supporting materials for hexacyanoferrates are porous materials, such as ion exchange resins, sawdust, wood-base materials, fiber pulp materials, paper, natural fibers or regenerated cellulose fibers. Enrichment is always purely surficial. The uptake capacity of such supporting materials for hexacyanoferrates is limited by their surface area and pore structure.
It is known to use compounds from the group of hexacyanoferrates to remove heavy metal ions of the elements cesium, strontium, thallium, cadmium and lead from aqueous systems. Hexacyanoferrates bind the recited heavy metal ions by ion exchange and incorporation in their ionic lattice. The group of hexacyanoferrates is made up of electrically neutral compounds which are comprised of the negatively charged hexacyanoferrate anion and positively charged counter-ions, preferably metal cations.
Preference among this group is given to the use of representatives which are sparingly soluble in water. Typical examples are Prussian blue Fe4[Fe(CN)6]3 or AxBy[Fe(CN)6] (A=NH4/Li/Na/K, B=Fe/Ni/Co/Cu/Ti/Cr). The patent DE 3735304 and the IAEA Report “The use of Prussian Blue to reduce radiocaesium contamination of milk and meat produced on territories affected by the Chernobyl accident” of February 1997 describe the use of hexacyanoferrates to bind Tl+ and Cs+ isotopes in human and veterinary medicine. The orally administered hexacyanoferrates are secreted again by the body. These uses require no separation or supportation, so pure hexacyanoferrates can be used. When hexacyanoferrates are used to remove heavy metal ions from aqueous systems, however, it is desirable for these to be completely separated from the system. The insoluble microcrystalline hexacyanoferrates such as NH4Fe[Fe(CN)6] or Fe4[Fe(CN)6]3 form colloidal solutions in water.
Colloidal particle sizes between 1 and 100 μm and below 1 μm limit the use of pure hexacyanoferrates in filter layers. Filter layers comprising hexacyanoferrates of this type are associated with substantial pressure drops over the length of the filter bed. The colloidal solubility further leads to a drag-out of the hexacyanoferrate even through barriers having pore sizes of less than 0.1 μm.
EP 0 575 612 A1 describes porous, particulate or fibrous supporting materials which have been treated with a suspension of hexacyanoferrates. The supporting materials thus treated bind radioactive cesium, rubidium and strontium ions in particular. The attempt to load cellulose fibers, especially lyocell cellulose fibers, with hexacyanoferrates revealed several disadvantages. The fibers could only be loaded with a limited amount of hexacyanoferrates. The hexacyanoferrate particles moreover displayed low adherence and so were easily washed off. The poor adherence of the hexacyanoferrate particles to the cellulose fibers moreover led to dusting in the course of processing. The distribution of the hexacyanoferrate particles on the fibers was also not homogeneous.
The problem addressed by the present invention is therefore that of incorporating hexacyanoferrates such as NH4Fe[Fe(CN)6] or Fe4[Fe(CN)6]3 in a matrix, ideally without impairing their ability to adsorb or absorb particular heavy metal ions, and thus preventing the formation of colloidal solutions. The incorporated hexacyanoferrates shall be firmly attached in the matrix and form a homogeneous distribution therein. The corresponding shaped articles shall be useful as filter material having high uptake capacity for heavy metal ions.
Various methods and solutions have been described for this problem, which all have disadvantages.
One proposed solution to the stated problem is to fix hexacyanoferrates to ion exchange materials. Thus, EP1419009 utilizes a composite material based on a supporting material having a coating of ion exchange material to effect ionic fixing.
RU2033240 further describes binding hexacyanoferrate to viscose fibers with ion exchanger functionalization. Even porous natural materials such as sawdust are stated by EP575612 to be capable of sorbing and hence binding hexacyanoferrates. With ionic fixation, displacement off the ion exchanger by other ions is possible. Surficially coated porous supporting materials, by contrast, are known to have limits with regard to adherence and mechanical stability.
A further approach to solving the stated problem is based on the idea of insoluble hexacyanoferrates being generated, and hence fixed, in the pores of porous supporting materials. This method is adopted in U.S. Pat. No. 5,601,722. Disadvantages here are that the method needs several individual steps for the synthesis and that the binding to the support is purely mechanical only. A further disadvantage is that the sorption kinetics of heavy metal ions are constrained by the degree of accessibility to the inner region of the pore structure.
Accordingly, there continues to be a need for shaped cellulose articles for selective binding of monovalent heavy metal ions, especially thallium and cesium ions and radioactive isotopes thereof.